Thymidylate synthetase is the target enzyme for the important cancer chemotherapeutic drug 5-fluorouracil. The nature of this enzyme and the interaction of this enzyme with 5-fluorouracil and other drugs has been studied extensively utilizing enzymes isolated from the bacterium Lactobacillus casei. It is the objective of this research to identify a mammalian source of thymidylate synthetase containing sufficient quantities of the enzyme to make purification of relatively large quantities practical. Affinity chromatographic approaches are to be developed and used for purification. The stability of complexes formed between the enzyme, fluorodeoxyuridine monophosphate and methylene tetrahydrofolate will be investigated using both equilibrium and kinetic techniques.